The present invention relates to a semiconductor laser device, a semiconductor laser device manufacturing method, an optical disk apparatus and an optical transmission system and relates, in particular, to a semiconductor laser device for use in an optical disk apparatus and an optical transmission system and a manufacturing method therefor.
A ridge waveguide semiconductor laser device, which has a ridge portion on its active layer and is able to be manufactured through one-time crystal growth process, has an advantage that the semiconductor laser device can be manufactured at low cost since the manufacturing processes thereof are simpler than those of a ridge-buried semiconductor laser device that needs three-time crystal growth processes.
FIG. 7 shows a prior art example a particular one of the ridge waveguide semiconductor laser devices in which current constriction is effected by a Schottky junction manufacturable at lower cost (refer to, for example, JP 04-111375 A) in comparison with the ridge waveguide semiconductor laser device for effecting current constriction by an insulator generally known as an air ridge type.
The semiconductor laser device is manufactured as follows. First of all, as shown in FIG. 7, an n-InGaP cladding layer 402, an InGaAs/GaAs strained quantum well active layer 403, a p-InGaP cladding layer 404 and a p-InGaAs contact layer 405 are successively layered on an n-GaAs substrate 401 by the metal-organic chemical vapor deposition (MOCVD) method. Next, the p-InGaP cladding layer 404 is etched partway by the technique of photolithography or the like to form a mesa. Thereafter, the materials of Ti/Pt/Au are deposited successively from the lower layer as a p-electrode 406, and the materials of Au—Ge—Ni/Au are successively deposited as an n-electrode 407. When a current is flowed through the thus manufactured device, a Schottky junction portion 408 is formed between the p-InGaP cladding layer 404 and the p-electrode 406, and a current flows only between the p-electrode 406 and the p-InGaAs contact layer 405, effecting the current constriction.
In contrast to the manufacturing method of the ridge-buried semiconductor laser device that has needed three-time crystal growth processes, the construction of the conventional ridge waveguide semiconductor laser device can be manufactured through only one-time crystal growth process. Furthermore, since an insulator or the like is not used for current constriction, the manufacturing processes are largely simplified, and a ridge waveguide semiconductor laser device, which can be manufactured at an overwhelmingly low cost in comparison with the conventional ridge-buried semiconductor laser device, is provided.
However, the ridge waveguide semiconductor laser device has had the following problems. That is, in the ridge waveguide semiconductor laser device in which the current constriction is effected by the Schottky junction without forming an insulator as in the prior art example, the electrode is put in direct contact with the side surfaces of the ridge portion and the surface of the cladding layer extended outwardly of the ridge portion dissimilarly to the air ridge semiconductor laser device in which the current flow is prevented by providing an insulator at the interface between the electrode and the semiconductor layer. It has been discovered that the emission laser light distribution disadvantageously comes to have a configuration such that it is easy to leak toward the electrode formed on the ridge portion side surfaces and the surface of the cladding layer in the neighborhood of the ridge portion depending on the relation between the refractive index of the semiconductor material that constitutes the semiconductor laser device and the refractive index of the electrode material.
For example, with regard to the refractive indices of the electrode materials used in the conventional ridge waveguide semiconductor laser device, the refractive index of Ti brought in contact with the semiconductor material is about 3.0 to 3.6 for a laser having wavelength from 600 nm to 1.5 μm, and the refractive index of Pt provided on the upper side of Ti is 3.0 to 5.5 for a laser having wavelength from 700 nm to 1.5 μm. On the other hand, an effective refractive index in the vertical direction outside the ridge portion is also, for example, about 3.2, and the refractive index values of them are close to one another.
As described above, if the effective refractive index in the vertical direction outside the ridge portion and the refractive index of the electrode material formed directly on the semiconductor layer have the values close to each other, then light sometimes tends to easily leak in the direction toward the electrode in the conventional ridge waveguide semiconductor laser device in which the current constriction is effected by utilizing the Schottky junction.
Then, the discovered problem is that the metal material constituting the electrode became a very large absorption component of the oscillating laser light to increase the internal loss, possibly causing an increase in the oscillation threshold current of the semiconductor laser device and reducing the slope efficiency when light disadvantageously leaks to the electrode since the photoabsorption coefficient of the metal material is generally 104 to 105 times as high as that of the semiconductor material.